1. Field of the Invention
The invention relates generally to cathode ray beam deflection systems and more particularly to display systems with electromagnetically deflected cathode ray tube random stroke and periodic raster displays. More particularly, the invention relates to systems of the type including dual mode deflection amplifiers providing increased deflection voltage for reducing slewing time in a stroke mode and for reducing power consumption during a raster mode.
2. Description of the Prior Art
The power efficiency of deflection systems that display both raster and stroke writing is relatively low due to the high yoke driving voltages required to assure adequate writing speed. There is a significant increase in power consumption as sophisticated airborne navigation displays require increased display area and more displayed information. Since the deflection yoke driver consumes a significant portion of the total display power, it is desirable to enhance the power efficiency of the deflection system.
A primary requirement for deflection amplifiers for an electromagnetically deflected CRT is that of supplying accurately controlled currents to the deflection yokes to assure linear operation and to provide minimum power dissipation. A CRT deflection yoke is an inductor, hence the supply voltage required is determined by the desired deflection driver output voltage, L di/dt+iR. Since the rate of deflection for the raster display is generally much higher than for stroke deflection, the supply voltages for raster deflection are correspondingly higher. Further the greater the amount of information to be presented by the display during the stroke period, the higher the slew rate required and the greater the peak power required. Power reduction in the prior art was limited to switching the yoke driver supplied voltages to a lower level during the stroked deflection period than during the raster deflection period, and the use of a flyback raster during the retrace. Apparatus providing suitable circuitry for this purpose is shown in U.S. Pat. No. 3,965,390, issued June 22, 1976 to James M. Spencer, Jr., and assigned to the assignee of the present invention.
With a flyback raster, a period of time is required for flyback and beam settling for each raster line. This time may be as much as ten percent of the total raster time. The power required for flyback is essentially wasted power. Further, higher supply voltages are required to reduce the flyback time, resulting in increased power consumption. By utilizing a bi-directional raster (i.e., writing alternate lines of raster in opposite directions), additional information may be displayed during the retrace period without increasing the supply voltages.
The present invention describes a system for reducing power during the raster display as well as during the stroke display. The invention differs from the prior art by switching the power supplied to the yoke driver during the raster deflection period to the minimum power level required by the deflection waveform, as well as switching supply levels between the raster and stroke deflection periods. The prior art invention of Spencer cited above switches power supplies during the stroke period, but does not augment efficiency during the raster deflection. The present invention is useful in both flyback raster systems and bi-directional raster systems.